A thin prism P_1 with angle 4° and made from glass of refractive index 1.54 is combined with another prism P_2 made from glass of refractive index 1.72 to produce dispersion without deviation…

Q: A thin prism P1 with angle 4° and made from glass of refractive index 1.54 is combined with another prism P2 made from glass of refractive index 1.72 to produce dispersion without deviation. What is the angle of the prism P2 ?

Sol: In case of thin prism δ = (μ-1)A ,

When two prisms are combined together,

δ = (μ-1)A + (μ’-1)A’

For producing dispersion without deviation

0 = (μ-1)A + (μ’-1)A’

(μ-1)A = – (μ’-1)A’

(1.54-1)4° = – (1.72-1)A’

A’ = -3°

So the angle of the other prism is 3° and opposite to the first.